208 THE GEOLOGY OP MINNESOTA. 



[Diabase. 



One section examined. 



Age. Cabotian. 



Remark. This curious and interesting rock is but a local phase of contact 

 between the basic and the acid magmas, which seems to have mingled without entire 

 chemical union. The field notes (Annual Report, ix, page 34), show the confusion 

 which prevails along the coast at this point and for some distance. The action of 

 the great diabase sheet, commencing at Silver creek and extending continuously at 

 least to this point, amongst the "red rocks" of the Cabotian, affected them profoundly, 

 and produced in the acid rocks, in some places, a condition of fluidity, from which 

 sometimes they crystallized into granites, but more frequently, perhaps, were left 

 but slightly changed. (Compare No. 132A.) See Part III, for a further discussion of 

 this subject. 



Wadsworth's No. 136 (Bulletin ii, page 125) is not this rock, but is misprinted 

 for No. 736. His No. 134 should read No. 734, and No. 153 should be No. 753. 



N. H. W. 



No. 137. DIABASE. (Modified.) 



From the same place as No. 136, i. e., near the centre of sec. 28, T. 56-7, but having the green and red colors 

 in larger patches, and therefore more strongly contrasted. 



Ref. Annual Report, ix, page 35; Bulletin ii, page 109. 



Meg. The green portions of the rock have the appearance of a coarse diabase, 

 in the interstices of which is seen a fine, green, amorphous substance, some of the 

 patches of this substance being half an inch in diameter and excluding all the crys- 

 talline elements of the rock. The red portions have the appearance of a red granite 

 in the interstices of which is seen an amorphous red substance which is more abun- 

 dant in some places than in others. This red substance appears like the aporhyolitic 

 magma of No. 140(7) and also like that of No. 140(2) and of several other of the red 

 rocks that occur in the vicinity of Beaver bay. The only noticeable difference between 

 this red substance and those mentioned consists in a brighter redness and more friable 

 texture of this, making it approach more nearly the appearance of a powder of com- 

 mon ferric oxide. Quartz is seen in both portions of this rock, but it is more abundant 

 in the red. Magnetite is abundant in the green and is not wanting in the red. In 

 short, the chief cause of the difference in color between these parts of this rock seems 

 to lie in the difference in the nature of the ferric oxide. This again seems to imply 

 a difference in the degree of basicity. At the same time, in the absence of olivine, 

 it would be allowable to attribute some part of this green element to the decay of 

 that mineral. 



Mic. The green portion of this rock consists of most of the essentials of a coarse 

 diabase. It only lacks olivine. The feldspars are striated and frequently zoned, and 

 sustain a curious relation to the pyroxene. The latter mineral is in general older 



